1. Field of the Invention
The present invention relates generally to an apparatus and method for improving the reception performance in a smart antenna system, and in particular, to an apparatus and method for improving the reception performance of a Mobile Station (MS) by canceling interference from a neighbor Base Station (BS) that affects the reception performance of the MS in a smart antenna system.
2. Description of the Related Art
The performance and capacity of a mobile communication system are basically limited by the features of radio propagation channels, including co-channel interference and path loss between cells or within a cell, multipath fading, signal delay, Doppler spread, and shadowing. To make up for the performance and capacity limits, mobile communication systems adopt techniques such as power control, channel coding, rake reception, diversity antenna, cell sectorization, frequency division, and spectrum spread.
With the advent of the wireless multimedia era, the demand for high-speed transmission of a large volume of data on radio channels is ever increasing. High-speed data transmission requires high transmit power and a wide transmission bandwidth, compared to general data transmission. Accordingly, the effect of high-speed data has to be compensated for in a hybrid cell environment with a variety of service signals. A promising solution for commercialization to performance degradation caused by interference and channel characteristics is smart antenna technology.
FIG. 1 shows a beam pattern in a typical smart antenna system. A BS 100 forms the beam pattern 111 of a pilot signal so the beam pattern 111 may cover the cell area 113 of the BS 100 in the smart antenna system. As to the beam pattern 115 of a traffic signal, the BS 100 forms it to direct only to an MS 102. When sending a traffic signal to the MS 102 within the service area 113, the BS 100 estimates the direction 117 of the MS 102 and steers a beam for the traffic signal to the MS 102. The beam pattern 115 of the traffic signal is narrowed to thereby reduce transmit power.
The transmission path between the BS 100 and the MS 102 is defined by radio resources. A downlink signal from the BS 100 to the MS 102 may reach the MS 102 directly along the direction 117, but typically it is subject to multipath fading 119. That is, it experiences reflection, refraction and scattering according to the surrounding topography before arriving at the MS 102. If the signal is spread in other adjacent directions as well as in the direction of the MS 102, this phenomenon is called angular spread.
If the beam pattern 115 of the traffic signal does not include all the angular spread, the traffic signal and the pilot signal arrive at the MS 102 from different propagation paths. As a result, the traffic signal differs from the pilot signal in phase. The MS 102 compensates for the phase of the traffic signal using the pilot signal and the difference in phase leads to a considerable decrease in the reception performance of the MS 102. Accordingly, the BS 100 has to form the beam pattern 115 including the angular spread so the traffic signal and the pilot signal have the same phase.
FIG. 2 shows typical smart antenna system according to the prior art. A first BS 200 (BS 1) forms a traffic beam pattern 211 for a first MS 204 (MS 1) within its cell area, and a second BS 202 (BS 2) forms a traffic beam pattern for a second MS 206 (MS 2) within its cell area.
When MS 1 is located at a cell boundary between the two BSs 200 and 202, i.e. in a handover region, the strength of a signal from the serving BS 200 becomes weak at MS 1, and increasing interference from the neighbor BS 202 leads to a decrease in the Carrier-to-Interference Ratio (C/I) of MS 1. MS 2 is also subject to the same effect as MS 1. The C/I is expressed by Equation (1)
                              C          ⁢                      /                    ⁢                      I            MS                          =                              S            sBS                                              I              sBS                        +                          I              nBSl                        +            …            +                          I              nBSm                        +                          N              o                                                          (        1        )            where SsBS denotes the signal strength of the serving BS, IsBS denotes the internal interference of the serving BS, InBSm denotes interference from an mth neighbor BS, and No denotes noise.
As described above, when the MS is located at the cell boundary between the serving BS and the neighbor BS, the C/I of the MS is decreased by interference from a neighbor BS, thereby degrading reception performance.